Hoists commonly have slip clutches in the drive train in order to prevent an overloading of pulling means or elements, gearing or other components of the hoist, or of supporting structures that hold the hoist. Also, in case of an operating error such as, for example, when a load becomes hung up on an obstacle when the load is being moved upward, damage to the chain pulley block, or any other danger situations must not occur. This also applies when the hoist is used in a horizontal operating mode, for example, for driving a chassis or a carriage.
U.S. Pat. No. 3,573,517 discloses a hoist having a drive train comprising a motor, a gearing, and an interposed slip clutch arrangement. A brake is provided on the gearing side, the brake being configured as a so-called load pressure brake. The load applies a torque to a gearing output shaft via a pulling means and its winding wheel. This torque is used to displace the brake shoes of the brake into the direction of engagement so that the load continues to be held safely while the motor is being deenergized.
The slip clutch arrangement comprises a parallel arrangement of a hysteresis clutch and an overrunning clutch. When the load is being moved upward, the brake is released and the overrunning clutch is inactive. The torque transmitted by the hysteresis clutch is greater than the nominal load torque so that the load is moved upward in a non-slip, positive manner.
When the load is being lowered, the overrunning clutch engages. The clutch effects a non-torsional connection between the motor and the gearing input shaft. Now, by means of the force of the motor, the holding torque of the load pressure brake is overcome and the load is moved downward. Consequently, the hysteresis clutch is disposed to act as a safeguard against overloading. If a constant slippage occurs on said clutch, a thermoswitch is actuated in order to stop the drive.
In addition to loads being hung up, it is possible for other disadvantageous operating situations to occur, thus requiring that such conditions be prevented or that its effects minimized. For example, due to resonance stimulation as a result of the polygonal effect of the chain wheel, it is possible for a chain vibration to occur, with the vibration overloading or wearing the supporting structures, structural components or even the gearing of the chain pulley block. The occurrence of such vibrations must be counter-acted.
Furthermore, during switching on and switching off operations, as well as when the rotational speed is changed in the case of pole-changeable mains-controlled motors, rotational speed surges occur on the motors, with such surges potentially leading to a shock-like stress on the pulling means and the gearing. This also must be avoided.
In particular in the case of inverter-controlled drives, high safety requirements must be satisfied in the construction of inverters. In any event, the occurrence of uncontrolled movement of a load if the inverter is defective or its control is malfunctioning must be prevented. In this case, measures are desirable that can minimize or prevent any danger to man, machine and environment with great reliability and at low cost.